A general view shows damaged buildings in Aleppo city at night April 11, 2015. Photo: Reuters/Rami Zayat

Russia announced a last minute 48-hour ceasefire for Syria's war-torn Aleppo late Wednesday, just hours after Washington warned Syrian President Bashar al-Assad and his ally Moscow that they must respect the nationwide truce agreed in February.

The vital northern province of Aleppo has been ravaged on multiple fronts in a devastating war that has killed more than 280,000 people.

Dozens of fighters were killed in a fresh bout of fighting between the regime, rebels, and jihadists south of Aleppo city on Wednesday, said the Syrian Observatory for Human Rights.

Backed by Russian and government air strikes, pro-regime fighters are locked in battle with rebel groups and Al-Qaeda affiliate Al-Nusra Front for a string of villages lying in hilly terrain between strategic routes.

"On Russia's initiative, a 'regime of silence' has been introduced in Aleppo for 48 hours from 00:01 16 June (2101 GMT Wednesday) with the goal of lowering the level of armed violence and stabilising the situation," the Russian defence ministry said in a statement.

The statement did not specify who Russia has discussed the two-day ceasefire with.

It accused Al-Nusra of attacking various Aleppo neighbourhoods with multiple rocket launchers, as well as mounting a tank attack southwest of the city.

The five-year conflict has drawn in world powers who back opposing sides -- including the United States which broadly supports the opposition and Russia on the side of Assad.

"Russia needs to understand that our patience is not infinite, in fact it is very limited with whether or not Assad is going to be held accountable," said US Secretary of State John Kerry on Wednesday.

"We also are prepared to hold accountable members of the opposition," he said after a meeting in Norway with his Iranian counterpart Mohammad Javad Zarif.

UN-hosted peace talks aimed at ending the conflict have been stalled since April, and a fragile ceasefire deal between the government and non-jihadist rebels has all but collapsed.

"It is very clear that the cessation of hostilities is frayed and at risk and that it is critical for a genuine cessation to be put in place," said Kerry.

Efforts were underway, he said, to reach a new agreement "in the next week or two" to reinstate the ceasefire across Syria, leading to more humanitarian aid deliveries and a resumption of the peace process.

Fresh Aleppo clashes

Since fighting erupted in Aleppo province on Tuesday, at least 70 fighters in total have been killed and the villages of Zaytan and Khalasa have changed hands twice.

The strategic border province is criss-crossed with supply routes for various sides of the conflict, including rebels, regime, Kurds, and jihadists such as the Islamic State group.

Aleppo was once Syria's commercial powerhouse, but it has been a battleground since 2012 when rebels seized the east of the city confining the army to the west.

A hospital in eastern Aleppo supported by Medecins du Monde was heavily damaged in an air strike on Tuesday, the France-based charity said, without reporting casualties.

Three other hospitals in Syria were hit in bombing at the start of June, leaving 10 dead.

As opposing forces close in from either side, residents of both halves of the city fear a potential total siege on the northern metropolis.

The UN says nearly 600,000 Syrians live in besieged areas, most surrounded by government forces.

Earlier this month, it said the government had granted preliminary aid access to 15 of 18 besieged areas, after one was taken off the UN's list.

Dozens of opposition activist groups accused the UN of "capitulating" to Damascus on aid access.

The scathing report, authored by The Syria Campaign (TSC) advocacy group, was based on testimonies from current and former UN staff and other aid workers.

It accused the UN of "choosing to prioritise cooperation with the Syrian government at all costs," allowing the regime to unduly influence UN aid strategy.

The UN humanitarian coordinator for Syria, Yacoub El Hillo, said however that while aid access was not ideal, the UN continues to "assist Syrians based on need".

TSC spokeswoman Bissan Fakih countered: "A UN with the backbone to stand for its principles would help get aid to hundreds of thousands of Syrian civilians under siege, many of them only a few minutes' drive from where the UN is based in Damascus."